communal roosts as structured information centres in the raven, corvus corax

Journal of Animal Ecology

2003

72

, 1003–1014

© 2003 British Ecological Society

Blackwell Publishing Ltd.

Communal roosts as structured information centres in the raven,

Corvus corax

JONATHAN WRIGHT, RICHARD E. STONE and NIGEL BROWN

School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK

Summary

1.

Ravens (

Corvus corax

, L.) feed on rich but ephemeral carcasses of large animals. Non-breeding juveniles forage socially and aggregate in communal winter roosts, which mayfunction as ‘information centres’ regarding food locations.

2.

In a large roost in North Wales, regurgitated pellets on the forest floor contained avariety of prey remains, which were more similar for ravens that had roosted closetogether the same night.

3.

Sheep carcasses placed at varying distances from the roost were baited with colour-coded plastic beads. These were ingested and regurgitated in pellets back at the roost inaggregations, the spatial distribution of which consistently reflected the geographicallocation of bait sites.

4

Aggregations of beads at the roost grew daily with an increasing radius centred uponthe first pellet per carcass. This mirrored the linear increase of six birds per day in the sizeof groups flying between roost and carcass each morning. Rates of recruitment weregreater for carcasses closer to the roost.

5

Groups were led by a single bird roosting centrally within the aggregation. Whenindividually identifiable (37·5% of cases), these individuals were dominant at the carcassand were among the minority of birds involved in acrobatic display flights at preroostgatherings.

6

When contrasted with data on two alternative groups of ravens peripheral to the mainroost which foraged and roosted collectively, these results provide strong circumstantialevidence for raven roosts as structured information centres. The adaptive basis forcompetitive recruitment resulting in excessively large group sizes is also discussed.

Key-words

: information centre hypothesis, local enhancement, recruitment, scaveng-ing, social foraging.


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Introduction

Communal roosts and breeding colonies were firstproposed by Ward & Zahavi (1973) as adaptations forexploiting ephemeral but locally abundant food sources.They suggested that roosts and colonies serve as‘information centres’, whereby birds that have foragedunsuccessfully can learn the whereabouts of food sourcesby following successful foragers the next morning fromthe roost or colony. Early evidence appeared to supportthe hypothesis, for example from studies on the greatblue heron (

Ardea herodius

, Krebs 1974) and the sandmartin (

Riparia riparia

, Emlen & Demong 1975).

However, Mock, Laney & Thompson (1988) suggestedthat many of these results could be explained by otherforms of social foraging. For example, local enhance-ment involves foraging birds locating food by detectingbirds already feeding at a patch (e.g. Poysa 1992), andso roosts may function as simple assembly points andto spatially concentrate foragers at the start of eachday (‘recruitment centres’, Richner & Heeb 1995,1996).

Despite the mixed history of this idea, experimentalevidence now exists for communal roosts acting asinformation centres in two species of New Worldvultures (black and turkey vultures,

Coragyps atratus

and

Carthes aura

: Rabenold 1987; Buckley 1996, 1997).Even more convincing is the extensive evidencefrom Western Maine, USA, showing that small groupsof juvenile ravens (

Corvus corax

) use their mobileand ephemeral roost sites as information centres

Correspondence: J. Wright, School of Biological Sciences,University of Wales, Bangor, Gwynedd LL57 2UW, UK.Tel. +44 (0)1248 382313; Fax: +44 (0)1248 382313; E-mail: [email protected].

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(Heinrich 1988, 1990, 1994; Heinrich & Marzluff1991; Marzluff & Heinrich 1991; Heinrich, Marzluff& Marzluff 1993; Marzluff, Heinrich & Marzluff 1996).The advantages of cooperative recruitment are obvi-ous for both vultures and ravens, because they feedon large mammal carcasses that can be widely distri-buted and unpredictable in time and space. In addition,local territorial pairs of ravens can successfully defendany carcass from one or two juveniles, but give wayonly once six or more juveniles gather together(Heinrich 1990; Marzluff & Heinrich 1991; Heinrich

et al

. 1993). Upon discovering a large defended car-casses, juvenile ravens ‘yell’ to recruit other juveniles,and these additional birds are also attracted to theappeasement calls of birds attacked by territorial adultsat carcasses (Heinrich

et al

. 1993). However, recruitmentto carcasses occurs most effectively via communalroost sites. The best evidence for this comes fromMarzluff

et al

. (1996), who found that when naïvebirds (held captive for varying periods of time) werereleased at roosts, they followed knowledgeable birdsto carcasses. However, when these birds were releasedaway from roosts, they were very rarely sighted at suchcarcasses. Like a similar recent study on hooded crows(

Corvus corone cornix

: Sonerud, Smedshaug, & Bråthen2001), Marzluff

et al

. (1996) provides good experi-mental evidence for information exchange at com-munal raven roosts. Dominant knowledgeable birdsalso appeared to initiate preroost soaring displays,potentially advertising their discovery of food, becauseit was these same birds that initiated the synchronizeddeparture of feeding groups from the roost.

Communal raven roosts in North America thereforeappear to function as information centres, in thatindividuals use them to actively recruit conspecificsand direct them to these food bonanzas (

sensu

Ward& Zahavi 1973). The evolutionary stability of sharingsuch foraging information with potential competitorshas been confirmed with formal game theory models(Mesterton-Gibbons & Dugatkin 1999; Dall 2002).Additional explanations involving kin selection andlong-term reciprocity have been ruled out, becauseraven roosts are not consistently made up of groupsof relatives (Parker

et al.

1994) or repeated coalitionsof the same individuals (Heinrich 1990). It has alsobeen postulated that recruitment behaviour may berelated to mate choice and an individual’s positionin the social hierarchy (Heinrich 1990). However, itremains to be seen whether recruitment effort in ravensconveys an honest signal of individual quality (e.g. ofthe ability to find food and/or the cooperative propensityto share it).

In contrast to North America, raven roosts inEurope are far larger and more stable, probably as aresult of the birds foraging on more abundant foodand over much shorter distances in an agriculturallandscape (Ratcliffe 1997). However, almost no obser-vational or experimental data exist to show how suchlarge European communal raven roosts might function

as information centres. Here we explore this issueusing field observations of one of the largest ravenroosts in Europe, including the spatial distribution ofpellets regurgitated overnight by the ravens onto theforest floor. Experimental provision of sheep carcassesallowed baiting with plastic marker beads, whichappeared in the pellets back at the roost to showwhere and when birds had been feeding and roostingtogether. Daily counts were made of the number ofbeads per carcass, and backed up with observations ofgroups of birds leaving the roost and feeding at car-casses, some of which were individually identifiablefrom patterns of feather moult. Data were also col-lected from two coherent groups of ravens roosting andforaging locally but separate from birds in the mainroost. These ‘subroost’ groups provided an interestingcomparison with the results from the majority of birdsin the main roost.

Methods

Communal roosting and social feeding behaviour ofjuvenile non-breeding ravens was observed at a majorroost site in the Newborough forest (for annual vari-ation in the number of birds, see Fig. 1). Situated on thesouth-west coast of the isle of Anglesey, North Wales(4

°

20

′

W, 53

°

10

′

N), this raven roost has existed for atleast 14 years (Ratcliffe 1997). The forest is a 769-haconiferous plantation, planted between 1947 and 1965on sand dunes, and a central ridge of basaltic volcanicrock containing the trees where the majority of theravens roost. The roost is close to coastal areas ofdunes, cliffs and rocky shores, and extensive areas ofagricultural pasture on lowland Anglesey and uplandSnowdonia National Park, comprising largely ofunimproved sheep grazing (Fig. 2a).

The number of ravens gathering to roost at New-borough was recorded during most months betweenOctober 1995 and August 2000. This involved two to fivevolunteers at a time placed at convenient locationsaround the perimeter of the forest, each counting thebirds as they arrived from a separate direction. Thetotal number of birds recorded per count was thenreduced to an average per month whenever data wereavailable.

An initial survey was made of the forest floor under themain Newborough roost, and all the old pellets removedfrom eight separate 30

×

30 m

2

randomly placed quad-rats. For a 3-week period during 1998, all pellets fallingwithin the quadrats were collected in individual sealedplastic bags. Each pellet was then taken back to the

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laboratory, dried in an oven for 24 h at 60

°

C, the drymass taken (

±

0·01 g) and the contents identified (tospecies level where possible). These data were reducedto 11 classes of food: avian, avian shell, sheep, inor-ganic, rabbit, man-made, marine invertebrate, rodent,other small mammal, terrestrial invertebrate and veg-etable matter. The percentage content of each pelletrepresented by these categories was then estimated byvolume.

Thirty-one food bonanzas were provided, including 26sheep carcasses (

Ovis aries

, L.), one placement of a pairof brown hares (

Lepus europaeus

, Pallas), and fourplacements each comprising six grey squirrels (

Sciuruscarolinensis

, Gmelin). The carcasses were placed at fivesites around Anglesey and one on the North Walesmainland between August 1998 and February 2000, atdistances between 2 and 30 km from the main roost atNewborough (see Table 1, Fig. 2b).

Each sheep carcass was baited with 500 colour-coded plastic fishing beads (3–6 mm diameter) insertedinto scalpel incisions around the eyes, throat, neck,shoulder, back, haunches, thoracic and abdominalcavities. Brown hares were baited with 100 beads each,and grey squirrels with 20 beads each.

Each carcass was monitored daily, for 2 h in the morn-ing and 2 h in the afternoon using either (a) a hide andbinoculars at 20 m or (b) a telescope at over 50 m,depending upon the site, thus minimizing any distur-bance to the birds. Observations began when the car-cass was first put out and ended only when the ravensceased to feed upon it. The maximum number of ravens

at the carcass and/or on the ground within 15 m wasrecorded for each day. In addition, any distinguishingfeatures of individual ravens were recorded (e.g. miss-ing primary, secondary or tail feathers, which are verycommon in juvenile ravens), and used for individualidentification. The identity of consistently aggressiveand apparently dominant birds enjoying preferentialaccess to the carcass was also noted when available.

The entire forest floor below the roost was systemat-ically searched daily for up to 8 days after each carcasswas put out. Each new pellet found was broken apart tosee if it contained any of the bait beads, and if so its geo-graphical position was noted. The exact position wasrecorded for every subsequent pellet containing baitbeads, and distance measured (to the nearest cm) fromthe first pellet found per carcass.

Markers were placed in the tops of the trees denotingan area roughly twice the size of the area in which thebaited beads from each carcass were found. At dawnthe following morning, the number and identity (wherepossible) of all birds leaving that area of the forest wasrecorded (usually from a convenient tall tree at least200 m away). A coherent group usually left within 5 minof the first bird and all in the same recorded direction.

Each evening, preroost soaring displays were observedand the behaviour of any individually identifiable birdsrecorded whenever possible.

Variables were analysed using parametric tests onlywhen they conformed to homogeneity of variance andnormality requirements. Two-tailed

P

-values are giventhroughout.

Fig. 1. Annual patterns in the number of ravens estimated to be roosting overnight in Newborough forest between January andDecember each year from 1995 to 2000.

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Results

Figure 1 shows the change in observed numbers of ravensroosting each month at the Newborough site, from October1995 to August 2000. Numbers varied from a few hundredbirds in the summer to over 1500 birds in the winter. Thesize of the roost increased gradually during the mid-1990s,but since 1997 has remained stable and a regular annualpattern in the number of roosting birds has emerged.

The structure of the Newborough roost is shown inFig. 2(b). The main roost consisted of a large numberof unpaired juvenile birds. Two separate subroostswithin the same wood each contained up to 30 birds,almost all of which appeared to be paired. These pairstolerated each other and defended their communalforaging area immediately around Newborough wood,although no pairs nested within

∼

5 km of the New-borough roost in any of the years of this study. The sur-rounding area of North Wales (within 50 km radius ofNewborough) contains a dense population of 200–400breeding pairs of ravens, roosting and feeding within

Fig. 2. Maps of (a) the area of North Wales and Anglesey and the six carcasses locations; and (b) the Newborough forest ravenroost showing the extent of the main overnight roost and preroost gathering areas during the winter 1999–2000. Also shown in(b) are the locations of the nine sites within the main roost, and the two subroosts, within which pellets from bait carcasses werecollected (see Table 1).

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their own separate territories which are largely exclusive(J. Wright, R. E. Stone & N. Brown personal observa-tion; Ratcliffe 1997). The large numbers of unpairedbirds from the main roost therefore left each morningin loose groups to feed in and around the territories ofpaired birds along the extensive coastline, farmland andmoorland of the island of Anglesey and the uplands ofSnowdonia.

Table 2 shows the variation in the contents of 761natural pellets collected over a 3-week period at eightdifferent quadrats located within the main Newbor-ough roost (see Methods). Certain prey types were veryprominent, such as sheep, rabbit and rodent remains,and vegetable matter. Each of these often made up100% of pellet dry mass. Other prey types were muchrarer, but could represent a high proportion of somepellets, such as inorganic items (e.g. grit swallowed toassist digestion), man-made items (often scavengedfrom rubbish tips), and marine and terrestrial inverte-brates. Out of season prey types, such as avian remainsand avian egg shell, appeared very rarely and even thenonly in low percentages of the total dry mass of a pellet.

As might be expected from the geographical separa-tion of their respective feeding sites, the proportions ofdifferent prey types within raven pellets were often neg-atively associated. The proportion of sheep remainswas significantly negatively correlated with the propor-tion of rabbit (

r

=

−

0·30,

n

= 761,

P

< 0·001), vegetablematter (

r

=

−

0·15,

n

= 761,

P

< 0·001), rodents (

r

=

−

0·19,

n

= 761,

P

< 0·001), small mammals (

r

=

−

0·14,

n

=761,

P

< 0·001), and less convincingly to marine inverte-brates (

r

=

−

0·07,

n

= 761,

P

= 0·048). Rabbit remainswere significantly negatively associated with the pro-portion of rodent (

r

= 0·11,

n

= 761,

P

= 0·003), smallmammal (

r

= 0·11,

n

= 761,

P

= 0·003), and vegetablematter (

r

= 0·11,

n

= 761,

P

= 0·003). Marine andterrestrial invertebrate remains were significantly negat-ively associated (

r

=

−

0·17,

n

= 761,

P

< 0·001), aswere rodent remains and vegetable matter (

r

=

−

0·08,

n

= 761,

P

= 0·035). The only significant positive asso-ciations were between avian and inorganic materials(

r

= 0·11,

n

= 761,

P

= 0·003), which like the marginalassociation between rabbit and inorganic materials(

r

= 0·07,

n

= 761,

P

= 0·055) probably reflects ravensingesting road grit while scavenging on road kill.

Although pellet size (dry mass) did not differ signi-ficantly between the eight quadrats, Table 2 shows thatthere were a number of significant differences in prey

Table 1. Locations of the bait carcasses placed out containing coloured plastic beads, the distance (km), the six different datesof placement, and the area within the Newborough roost (see Fig. 2) in which pellets with beads in were recovered (R = mainroost; S = subroost). Locations close to the roost often received two carcasses at a time, and these are divided according to whetherthey were fed on by either main roost or subroost (sr) birds

Location Distance Date R1 R2 R3 R4 R5 R6 R7 R8 R9 S1 S2

Abergwygregyn 26·9 22/1/99 0 0 0 0 0 0 0 0 27 0 0Abergwygregyn 26·9 8/11/99 0 0 0 0 0 0 0 0 39 0 0Abergwygregyn 26·9 22/2/00 0 0 0 0 0 0 0 0 21 0 0Aberffraw 5·8 8/11/99 0 0 0 0 0 0 0 43 0 0 0Aberffraw 5·8 22/2/00 0 0 0 0 0 51 0 0 0 0 0Cors Erddreiniog 20·2 28/10/98 0 0 0 0 0 0 0 0 0 0 0Cors Erddreiniog 20·2 5/12/98 0 24 0 0 0 0 0 0 0 0 0Cors Erddreiniog 20·2 22/1/99 0 25 0 0 0 0 0 0 0 0 0Cors Erddreiniog 20·2 8/11/99 0 22 0 0 0 0 0 0 0 0 0Cors Erddreiniog 20·2 22/2/00 0 39 0 0 0 0 0 0 0 0 0Malltraeth 9·6 28/10/98 12 0 0 0 0 0 0 0 0 0 0Malltraeth 9·6 22/1/99 0 0 29 0 0 0 0 0 0 0 0Malltraeth 9·6 8/11/99 0 0 32 0 0 0 0 0 0 0 0Malltraeth 9·6 22/2/00 0 0 47 0 0 0 0 0 0 0 0Newborough 2·5 28/10/98 0 0 0 59 0 0 0 0 0 0 0Newborough 2·5 5/12/98 0 0 0 51 0 0 0 0 0 0 0Newborough 2·5 8/11/99 0 0 0 0 60 0 0 0 0 0 0Newborough 2·5 22/2/00 0 0 0 0 38 0 0 0 0 0 0Newborough 2·5 22/2/00 0 0 0 0 0 0 35 0 0 0 0Newborough sr 2·5 9/9/98 0 0 0 0 0 0 0 0 0 23 0Newborough sr 2·5 28/10/98 0 0 0 0 3 0 0 0 0 0 0Newborough sr 2·5 5/12/98 0 0 0 0 0 0 0 0 0 26 0Newborough sr 2·5 22/1/99 0 0 0 0 0 0 0 0 0 24 0Newborough sr 2·5 8/11/99 0 0 0 0 0 0 0 0 0 27 0Rhedyn Coch 2·9 22/2/00 12 0 0 0 0 0 0 0 0 0 0Rhedyn Coch sr 2·9 8/11/99 0 0 0 0 0 0 0 0 0 0 10Rhedyn Coch sr 2·9 9/9/98 0 0 0 0 0 0 0 0 0 27 9Rhedyn Coch sr 2·9 28/10/98 0 0 0 0 0 0 0 0 0 0 7Rhedyn Coch sr 2·9 5/12/98 0 0 0 0 0 0 0 0 0 0 8Rhedyn Coch sr 2·9 22/1/99 0 0 0 0 0 0 0 0 0 0 7

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type content, specifically in the percentage of sheep andrabbit remains, inorganic and vegetable material, andterrestrial invertebrates. All of these results hold despiteany correction applied to

P

-values due to multiple test-ing, but other marginal prey type results (e.g. marineinvertebrates) can probably be disregarded on thisbasis (Table 2). Ravens roosting close to each other hadtherefore been feeding on more similar types of preythan birds roosting further apart. This suggests somespatial structuring of the Newborough roost in accord-ance with recent foraging activity, possibly linked to thegeographical locations of different feeding sites.

There was a consistent delay of 2 days from the day ofcarcass placement to the day when the first pellet wasdiscovered at the roost containing a bait bead, pre-sumably reflecting 1 day to discover the carcass andanother to ingest and regurgitate the bead. Interestingly,for the more distant sites at Abergwyngregyn and CorsErddreiniog this delay was always 3 days. The extraday in these cases possibly represented the additionaltime it took for bait carcasses to be discovered, with theravens having to fan out over proportionally largerareas at greater distances from the roost.

Overall, 960 pellets containing bait beads wererecovered from the 30 carcasses placed at the six differ-ent locations. Of these, 837 were recovered at 11 local-

ized sites within the roost (see Table 1, Fig. 2b), 96 atfour different preroost assembly points and a further 27scattered randomly throughout the forest. This sug-gests that the vast majority of pellets were produced atthe overnight roost site used by each bird, and thatthese bait beads provided an accurate record of whereeach bird spent the night following a day spent foragingat one of the bait carcasses.

Table 1 confirms that beads from each baited carcassnearly always appeared at only one specific site withinthe roost. Figure 2 shows that these specific roostingsites reflected the geographical distribution of carcasslocations. Pellets containing beads from carcasses tothe north-west and north-east were found at siteswithin the northern part of the roost, and beads fromthe carcasses to the west and east being found in thesouthern part of the roost. Despite the many monthsbetween carcass placements at the same locations, theroost area within which the beads were found was rem-arkably consistent for each location. This long-termspatial structuring of the roost was especially evidentfor the more distant carcasses (e.g. Abergwyngregyn,Cors Erddreiniog), whereas for the medium distancecarcass locations (e.g. Aberffraw, Malltraeth) beadsfrom carcasses at different times were occasionallyfound within different roost areas. For the pairs ofcarcasses placed at locations very close to the roost (i.e.Newborough, Rhedyn Coch), beads could turn up in arange of areas within the main roost. In addition, beadsfrom one of these nearby pairs of carcasses were always

Table 2. The contents of 761 natural pellets from the eight different 30 × 30 m quadrats within the main Newborough raven roost(see Methods for details). Data (mean ± SE, maximum in parentheses, minimum = 0 in all cases) are shown for dry mass, andpercentage dry mass for each prey type. Results of Kruskal–Wallis tests (d.f. = 7) are shown as χ2 statistics and P-values

Pellet content Quadrat within roost Kruskal–Wallis

1 2 3 4 5 6 7 8 χ2 P

Dry mass (g) 1·80 1·86 1·81 1·85 1·81 1·50 1·70 1·61 12·66 0·081(0·17) (0·13) (0·14) (0·08) (0·25) (0·12) (0·07) (0·08)

% avian 1 0 0 0 0 0 0 0 11·20 0·130(60) (2) (0) (1) (0) (1) (4) (1)

% avian shell 0 0 0 0 0 1 0 0 4·15 0·762(20) (1) (30) (19) (0) (53) (35) (1)

% bovid (sheep) 46 31 29 51 61 61 45 33 44·42 <0·001(100) (98) (100) (100) (100) (100) (100) (100)

% inorganic (grit) 5 9 6 5 7 11 8 8 40·71 <0·001(40) (66) (73) (60) (53) (95) (95) (76)

% lagamorph (rabbit) 21 20 20 10 6 4 17 22 18·93 0·008(100) (100) (100) (99) (91) (68) (99) (98)

% man-made 0 0 0 0 0 0 0 1 3·65 0·820(0) (1) (0) (0) (0) (0) (95) (69)

% marine invertebrate 2 5 0 3 2 0 2 1 16·12 0·024(93) (88) (10) (97) (47) (0) (75) (77)

% rodent 12 13 9 6 6 11 8 7 8·32 0·301(100) (97) (99) (99) (98) (99) (98) (97)

% small mammal 2 3 4 5 1 1 4 6 12·20 0·094(39) (85) (95) (99) (11) (33) (97) (99)

% terrestrial invertebrate 0 2 0 0 1 0 1 0 66·50 <0·001(11) (53) (9) (5) (13) (1) (40) (11)

% vegetable matter 10 17 30 19 17 11 15 23 22·31 0·002(73) (97) (100) (100) (84) (88) (99) (100)

N = 55 66 81 100 30 39 220 170

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found exclusively in one or other of the subroosts,depending upon which side of the roost they wereplaced (i.e. subroost 1 to the south-west, and subroost2 to the north; Table 1; Fig. 2b).

Within the main roost, there was an increase in thenumber of pellets found containing bait beads for eachsuccessive day that the ravens foraged on the carcasses.Figure 3 shows that this increase only lasted until thefourth day, and then the number of pellets contain-ing beads started to decline. Presumably, after thefourth day the number of beads in the carcass becamedepleted (see below). However, a different patternemerged for carcasses fed on by the subroost birds.These produced the full complement of beads in sub-roost pellets right from the start (Fig. 3). The numberof bait bead pellets found in the subroosts also declinedafter only 3 days, possibly again reflecting depletion of

beads and/or bait, but in this case it was more rapidowing to the greater number of birds feeding on thesesubroost carcasses from the beginning.

By recording the exact location the first bait beadpellet found, and plotting the distance between it andeach successive pellet from that carcass, we were ableto replicate successive changes in size of roostingaggregations. Figure 4 shows that the mean distancebetween this first pellet and all subsequent pelletsincreased each day until day 4, and then decreasedagain. For pellets in the main roost, the best fit for thesedistance data is represented by a quadratic function(Fig. 4 and 2nd order polynomial: r2 = 0·991, P =0·001). However, this was largely due to the later deple-tion in pellet number, and so when considering only thefirst 4 days a logarithmic function actually provides thebest fit (Fig. 4; r2 = 0·975, P = 0·012). Presumably, thisnon-linearity reflects the fact that as each successivebird joined the aggregation, it was forced to roost inbranches and trees further and further away from thecentral bird(s). Within the subroosts no such patternwas observed (Fig. 4), either over the full 6 days or thelimited 4 days (i.e. none of a range of curves provided asignificant fit: P > 0·305 in all cases). This result prob-ably reflects the lack of order and structure in carcassuse and roosting position within subroosts, as com-pared to the structured and consistently expandingaggregations within the main roost.

Figure 5 shows that on each successive morning therewas an increase in number of birds leaving the specificmain roost areas and flying off together in the samedirection. The maximum number of birds observedfeeding on each carcass also increased on successivedays (Fig. 5).

Fig. 3. Mean number (± SE) of marked pellets from baitcarcasses per day found on the forest floor for the first 6 days.Data are split between clusters of marked pellets found withinthe main Newborough roost and the two subroost areas (seetext for further details).

Fig. 4. Mean distance (± SE) between the original pellet percluster and all other pellets from bait marked carcasses perday found on the forest floor for the first 6 days. Data aresplit between clusters of pellets found within the mainNewborough roost and the two subroost areas (see text forfurther details). Curves fitted to the main roost data include asecond order polynomial over all 6 days (solid line), and alogarithmic function over only the first 4 days (dashed line).

Fig. 5. Mean number (± SE) of birds leaving the main roost incoherent groups from the specific known areas of the roostand arriving at bait carcasses together per day for the first6 days. For comparison, data are also shown from Fig. 3 forthe numbers of marked pellets dropped by these birds onto theforest floor the night before in their specific areas.

1010J. Wright et al.

© 2003 British Ecological Society, Journal of Animal Ecology, 72,1003–1014

Figure 5 also shows the number of bait bead pelletsfound at the main roost (as in Fig. 3), which was alwaysless than the number of birds seen leaving those areas ofthe roost and present at the carcasses. Therefore, not allbirds produced pellets with beads in every night, andthe premature decline in the number of beads foundafter day 4 was almost certainly due to the early deple-tion of beads in the soft and accessible parts of the car-cass, rather than a real decrease in the number of ravensat each carcass.

Groups of birds leaving the roost could always beassigned to a carcass, based upon the colour of the baitbeads in pellets found in that specific area of the roost,and by the direction taken each morning, which alwayscorresponded to the location of the carcass in question.There was an extremely close correlation between thenumber of birds seen leaving a specific area of the mainroost and the number of birds observed arriving ateach carcass (Fig. 5; for 15 distant carcasses for whichdata were available: r > 0·98, n = 5, P < 0·001). Thisconfirms that it was the same groups of birds seen at bothlocations. However, there were always two additionalbirds at the carcass as compared with the group sizethat left the roost (Fig. 5; mean difference = 1·88 ±0·30; paired t4 = 6·19, P = 0·003), and this number of addi-tional birds did not differ significantly over time (Fig. 5;r2 = 0·419, n = 5, P = 0·237). Therefore, although thepresence of additional birds suggests the possibility oflocal enhancement, it probably reflects members of thelocal resident pair joining main roost members aroundthe carcass.

An interesting question concerns the pattern ofrecruitment of ravens to carcasses, as reflected by theincrease in numbers of birds leaving the roost on suc-cessive days. As Fig. 5 shows, this increased to anasymptote, because most carcasses had been almostcompletely consumed by day 5. By fitting a range ofcurves to the increase in raven numbers on days 1–4, itis clear that this increase is linear (Fig. 5; r2 = 0·99,n = 5, P = 0·006). This suggests that the rate of recruit-ment to the bait carcasses was constant at around 6·21birds per day, despite the ever-increasing numbers ofbirds who were aware of each carcass location. Inter-estingly, the intercept for this recruitment line on dayzero is close to −1 (i.e. −0·933). Along with the clear pat-tern of linear recruitment, this provides strong circum-stantial evidence to suggest that a single bird carriedout all of the recruitment starting on day zero. Inaddition, this linear rate of recruitment appeared todecrease the further the carcass was from the mainroost (r2 = 0·21, n = 15, P = 0·049), suggesting thatfewer birds were available and/or willing to be recruitedto more distant baits.

Further observations at the roost site confirmedthat only certain individuals were involved in recruit-ment behaviour, because a minority were recognizableby distinguishing characteristics (e.g. moult pattern)in 27 out of 72 occasions (37·5%). These birds appearedto be of central importance in the initiation of soaring

and ‘rolling’ flight displays that took place above thepreroost assembly areas each evening immediatelyprior to roosting (see Fig. 2b). These birds alwaysroosted on a central perch in the tree below which thefirst baited pellet from any given carcass was found.The same individuals also appeared to initiate morningdepartures from the roost, preceded by lots of vocali-zations. It was these birds that were the first to be seenat the carcasses on day one (five out of five cases wheredata were available). Therefore, these birds may wellhave been responsible for the recruitment, and on eachconsecutive day they were the first to feed at the car-casses (in seven out of seven cases where data wereavailable), appearing dominant over other ravens in theforaging group.

Discussion

The results presented here confirm the large and stablenature of the Newborough raven roost, which consistedof a hard core year-round resident group of 500 unpairedjuvenile ravens. The seasonal influx of large numbers ofbirds (>1000) appeared to be in the form of unpairedjuvenile birds from outside of North Wales. At presentit is unclear whether these two groups of resident vs.immigrant juveniles behave in similar ways, or whetherthey play different roles within cooperative roostingand foraging groups at Newborough. For example, resid-ent juvenile ravens may be more familiar with the localarea (or part of it) and possibly better at locating andrecruiting to carcasses, and perhaps therefore moredominant. In contrast, the seasonal immigrants may bemore socially parasitic and spend much of the winterbeing recruited to carcasses located by their roostmates.What is clear is that the raven roost at Newborough ishighly structured, based upon the geographical arrange-ment of potential foraging areas (see below), and thatthis must reflect social organization within the roost.

Observation of the changing numbers and identitiesof birds in the two subroosts during this study suggeststhat these birds were older paired non-breeders thatwere awaiting a suitable breeding territory. Indeed, cer-tain pairs appeared to leave the subroosts at the start ofthe 1999 breeding season, possibly as a result of findinga local breeding vacancy, or the decision to search fur-ther afield for a breeding territory. A similar pattern tothis exists in another communally roosting corvid, thechough (Pyrrhocorax pyrrhocorax) in Spain, wheresubroosts of older paired birds are thought to play arole in reducing the cost of mate and territory acquisi-tion (Blanco & Tella 1999). It therefore appears that thesubroosts at Newborough represent a half-way stagebetween mobile juvenile flocks and resident territorialpairs. These subroost birds collectively defended a feed-ing territory within which they foraged and roostedtogether as a cohesive group (see below).

1011Raven information centres


Natural pellet content reflected the varied diet ofravens roosting at Newborough, feeding in a range ofecological habitats. It was clear that birds that roostedclose together had been feeding on similar food itemsduring the previous day. However, it was not possible toidentify any specific geographical feeding areas fromthese clusters of similar pellet contents. This is becausethe full range of foraging habitats existed in every direc-tion from the roost, in the form of roads, rocky shore-lines and beeches, and grazing land. It was also clearthat most types of food remains appeared in at leastsome of the pellets recovered from within any one of thespecific area of the roost. The association of food typesfound in pellets dropped in the same area of the forestwas therefore probably a temporal phenomenon, andnot the result of different areas of the roost beingexclusively occupied by birds feeding only in one typeof foraging habitat.

The use of bait beads in carcasses confirmed the spa-tial structuring of the raven roost at Newborough, andthat precise roosting positions reflected the feedingsites being used. It was interesting to note that therewas a very consistent spatial organization within theroost according to the geographical area that thebirds were known to have been feeding that day. Birdsdiscovering carcasses in a particular location alwaysroosted in the same few trees, even when there wasmore than 12 months between carcass placements (seeTable 1). The reasons for this are not entirely clear. It ispossible that this reflects some form of social conven-tion by knowledgeable birds, with different parts of theforest traditionally representing different geographicalfeeding locations to any potential recruits. An altern-ative, and perhaps more likely, explanation is that thesame individual ravens tended forage and discover car-casses within the same geographical area, probablybenefiting from their local knowledge and experience.These different individuals may also have had theirown favourite roosting areas within the forest, therebylinking geographical locations with specific areaswithin the roost via their recruitment behaviour.

The notion of individual-specific roosting and for-aging areas also fits with the observation that morenearby bait sites were exploited by birds roosting in awider variety of areas within the forest. This wasalmost certainly the result of these sites being over-flown by a greater number of birds travelling to forageat more distant locations. Individual raven foragingareas therefore appeared to have been fan-shaped,spreading out from the roost in a specific and consist-ent compass direction. This also agrees with the obser-vation that the dedicated roosting areas within theNewborough roost were in locations that were closestto the distant foraging locations used by those birds(see Table 1 and Fig. 2). This pattern of roosting seemsunlikely to provide any meaningful reduction in dis-tances travelled, and may therefore reflect the historical

process by which the Newborough roost was formedfrom a series of smaller (and perhaps more mobile)raven roosts associated with specific foraging areas.

When the bait beads were used to confirm that groupsof pellets dropped on the forest floor below the roostwere produced by birds feeding at a particular carcass,there were two distinct patterns in the number and dis-tribution of pellets. In the main roost, the averagenumber of baited pellets increased linearly on a dailybasis until the carcass became depleted. Pellets herewere dropped in a tight cluster, which increased in sizeon consecutive days and centred upon the location ofthe first pellet(s) dropped. We suggest that this was aconsequence of naïve birds being successively recruitedby a central knowledgeable bird that dropped one ofthe first pellets within a cluster. The first naïve birdsroosted close to the knowledgeable bird, in the same ornearby trees, but each additional bird had to roost fur-ther and further away due to saturation of favourableperches closest to the knowledgeable individual. Suchobservations would be in accordance with expectationsfrom the information centre hypothesis (Ward & Zahavi1973; see Mock et al. 1988; Richner & Heeb 1995, 1996),and may provide new detail regarding the spatial patternsof roosting within groups of ravens during informationexchange and recruitment to a carcass.

In contrast to the main roost, within the subroosts alarge number of pellets containing beads was producedimmediately during the very first night, and then didnot increase after that. There was also no spatial struc-turing to the distribution of pellets dropped within sub-roosts on successive nights, suggesting no link betweenforaging and specific roosting locations. Subroost birdstherefore searched and foraged together as a group andprovide an excellent contrast to the recruitment andinformation exchange apparent in the main juvenileroost. Overcoming local competition for carcasses isclearly a more important factor closer to the roost,which in itself can drive recruitment behaviour andinformation exchange (see Mesterton-Gibbons &Dugatkin 1999; Dall 2002). Group searching and for-aging may represent an alternative strategy (see Dall2002) used by the subroost birds in highly competitiveareas close to the roost, and this is in contrast to theclassic information centre strategy of ‘search individu-ally and recruit’ used by main roost birds foragingover wider areas (J. Wright & S.R.X. Dall personalobservation).

The behaviours suggested by the patterns of bait pelletson the main roost forest floor were closely matched byobservations of individual ravens themselves, both at



the roost and at the carcass. Most importantly, when-ever it was possible to identify the individual ravensconcerned, there appeared to be only one dominantbird at the carcasses. These same individuals roosted inthe central tree below which the first baited pellet wasfound. These individuals also initiated the morningdeparture of groups of birds from the roost in the direc-tion of the carcass in question. All of these observationswere again consistent with requirements of theinformation centre hypothesis (see Mock et al. 1988;Richner & Heeb 1995, 1996) and match observationsof smaller raven roosts in Maine, USA (Marzluff et al.1996).

In agreement with Marzluff et al. (1996) and ourdata concerning pellet number (above), there was alinear daily increase in the number of ravens in groupsleaving the roost at dawn, as well as in the maximumnumber of birds feeding at the carcass that same day. Incontrast to the ‘all-or-nothing’ group foraging by sub-roost birds, this provides strong evidence for delayedrecruitment and information exchange at the roost.The fact that the intercept for these recruitment curveswas very close to −1 suggests that the initial recruitmentwas due to a single bird, presumably the individual thatfirst discovered the carcass. Recruitment would onlyhave remained so precisely linear on each successiveday after this if there had been no effect of localenhancement and recruitment at the roost was by onlythis one bird.

The consistent positive 1·88 bird difference betweenthe number of birds leaving the roost and the number atthe carcass is unlikely to have been the result of recruit-ment via local enhancement (e.g. two extra birds alwaysbeing attracted to the group each day on the way to thecarcass or at the feeding site itself ). Instead, these twoextra birds at the carcass were almost certainly the ter-ritorial adult pair. This is unsurprising since all baitsites were within 1·5 km of an active breeding site(Ratcliffe 1997). Resident territorial pairs of birds ini-tially defended newly placed carcasses against the firstjuveniles to discover these food bonanzas. Such carcassdefence probably explains the delay of 1 day betweenjuvenile ravens locating carcasses and the appearanceof the first bait pellets back at the roost, showing evid-ence of successful feeding. Access to carcasses wastherefore gained by juveniles recruiting about six birdson the first day, which appears to have been adequateto overcome the resident pair. This sequence of eventsprovides one adaptive explanation for such activerecruitment behaviour by the lead bird (i.e. the ‘posse’effect, Heinrich 1994; Marzluff et al. 1996).

The need for a ‘posse’ to overcome defence of foodbonanzas by resident pairs does not, however, explainwhy ravens in this study continued to recruit up to fivetimes the number of birds necessary to gain access tocarcasses. Each individual that was recruited above andbeyond the critical group size of six birds would seem tocarry a cost in terms of lost foraging opportunitiesfor the recruiting bird. It is possible that the cost of

excessive recruitment is actually relatively small due toan expected loss of access to many carcasses before allthe meat could be eaten by six birds. Carcasses can belost to ravens, either due to burial by sudden snowfallsor by consumption by other animals, such as canids orother corvids (Heinrich 1988). However, both of these pos-sibilities seem unlikely to occur in coastal North Wales,especially given current climatic conditions and the lownumbers of other carnivorous scavengers (at least rel-ative to the number of ravens). North Wales representsan artificial situation, with agricultural practices pro-viding rich feeding areas for ravens, and hence the veryhigh breeding densities and large permanent juvenileroost at Newborough. It is possible that the recruit-ment behaviour we currently see in ravens in NorthWales evolved in the past under very different condi-tions, perhaps more like those in North American studies(e.g. Heinrich 1988). Therefore, selection may have yetto act against the apparent maladaptation of excessiverecruitment behaviours in the context of highly agri-cultural ecology of the European landscape.

Alternatively, there may be current selection forexcessive recruitment by ravens in North Wales.Heinrich (1990) suggested that this behaviour hasevolved via some form of enhancement of social status,the idea being that ‘knowledgeable’ juvenile ravens thatdisplay and recruit at the roost are ‘showing-off’ toincrease their social prestige (see Zahavi 1995; Wright1999). By honestly advertising their fitness as futureproviders and useful collaborators, they increase theirchances of gaining a mate and reproducing successfullyin the future. This might explain why such ‘knowledge-able’ juvenile ravens played such a central role in theextravagant preroost flight displays, energeticallyrecruiting in competition with other birds, and whythese same birds maintained a proprietorial domin-ance at the carcass. However, more work is clearlyrequired to demonstrate the importance of recruitmentbehaviour in building social prestige and improvingfuture reproductive opportunities in juvenile ravens.

It is interesting to note that efficient informationtransfer at raven roosts could have been performed in amuch less metabolically costly manner (e.g. via simplevocalizations). For example, there was a conspicuouslack of such recruiting and information-sharing beha-viour in the already paired birds at the subroost sites.Excessively costly signals have been argued to evolve asa result of the need for honest information exchange inthe face of conflicts of interest between signaller andreceiver (Zahavi 1977, 1987; Grafen 1990; Johnstone1997). This suggests that recruiting ravens might havehad some interest in deceiving their audience, perhapsby exaggerating the profitability of the discovered car-cass in terms of distance and amount of food available.The fact that the rate of recruitment was slower to moredistant carcasses is consistent with the idea that pre-roost displays accurately reflected the energetic state ofthe displaying bird, and therefore the relative distanceand profitability of the carcass discovered. The more

1013Raven information centres


successful and rapid recruitment to carcasses placedcloser to the roost therefore appears to have been theresult of some form of market effect, with recruitingravens displaying in competition with each other at theroost each evening. It would be interesting to knowwhether it was the same individuals that were first tofind and recruit roostmates to successive carcasses,perhaps because they were the older, more experiencedresident birds. This would perhaps be one piece of evi-dence required to support the social prestige hypo-thesis (see above) suggesting that information exchangeand recruitment reliably signals some useful ability tofind and share food.

Conclusions

Our observations show for the first time that largestable European juvenile raven roosts operate as geo-graphically structured information centres. Using datafrom regurgitated pellets, the spatial organization ofraven roosts appears to correspond to the location andtype of food they were feeding on at the time. The useof baited carcass demonstrated that this spatial struc-turing was the result of recruitment behaviour byindividuals at the Newborough roost, matching obser-vations of smaller more mobile raven roosts in NorthAmerica (e.g. Marzluff et al. 1996). Birds that dis-covered bait carcasses recruited conspecifics using pre-roost flight displays, and spent the night surrounded bythe group that would follow them out the next morn-ing. Recruiting birds were dominant at the carcass anddisplayed in order to gather approximately six addi-tional birds per day until the carcass was depleted.Recruitment appeared to be competitive activity,which was more successful to geographically closercarcasses.

Taken with contrasting observations from the twosmaller subroosts of paired birds, these data supportrecent game theoretical approaches confirming theevolutionary stability of delayed recruitment and infor-mation exchange at such juvenile roosts (Mesterton-Gibbons & Dugatkin 1999; Dall 2002; J. Wright & S. R.X. Dall personal observation). However, additionalexplanations may be required to account for theexcessive numbers recruited to single carcasses inNorth Wales. Recruitment behaviour may provide anarena for juvenile ravens to ‘show off’ to potentialfuture partners. Alternatively, the apparently excessiverecruitment may be the result of rapid changes to theEuropean farming landscape making such informationcentre adaptations potentially maladaptive. Futurework is now needed to assess the impact of recruitmentbehaviour upon lost feeding opportunities at carcassesin this environment. We also need to know whether it isthe same subset of birds recruiting at the roost eachevening, either because they are the local resident juve-niles and therefore more knowledgeable, or becauserecruitment displays constitute reliable signals of indi-vidual food-finding and sharing abilities.

Acknowledgements

We are very grateful to the landowners who gave us per-mission to work on their land, including Forest Enterpriseand Countryside Council for Wales, and University ofWales, Bangor. Thanks also to the many observers ofthe Newborough roost over the years. For commentson earlier versions of this paper, thanks to Sasha Dall,Darryl Jones, Heinz Richner and Ron Ydenberg.

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Received 23 January 2003; revision received 10 June 2003

communal roosts as structured information centres in the raven, corvus corax

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